This invention relates to a code multiplexing wireless apparatus and, more particularly, to a code multiplexing wireless apparatus for generating a code-multiplexed signal, amplifying the code-multiplexed signal and then transmitting the amplified signal.
Wireless access using CDMA (Code Division Multiple Access) has been studied and is being put to use as the next generation of digital mobile communication. CDMA is a method of multiple access using spread-spectrum communication. Specifically, transmission information of a plurality of channels or users is multiplexed by coding and transmitted over a transmission path such as a radio link.
Spread-spectrum communication is a method of modulation that is different from ordinary modulation. In spread-spectrum communication, the bandwidth of a signal after modulation is made very large in comparison with that of the narrow band in modulation. With spread-spectrum communication, two-stage modulation/demodulation is performed in the transceiver.
FIG. 20 is a structural view illustrating the operating principle of a transmitter in spread-spectrum communication. Shown in FIG. 20 are a modulator 1 such as a (phase-shift keying) PSK modulator, a spreading circuit 2, a power amplifier 3 and an antenna 4. The positions of the modulator 1 and spreading circuit 2 may be interchanged. The spreading circuit 2 includes a spreading code generator 2a for outputting a rectangular spreading code sequence (see FIG. 21) that takes on levels of .+-.1, such as pseudorandom noise (PN), and a multiplier 2b for multiplying digital transmission data, which has been modulated by the modulator 1, by the spreading code.
As shown in FIG. 21, the speed at which the spreading code changes (namely duration Tc of the rectangular wave) is set so as to change over at a very high rate in comparison with symbol changeover speed (one bit interval T of the PSK-modulated signal) of the narrow-band modulated signal that is modulated by the spreading code. That is, T &gt;&gt; Tc holds. The duration of T is referred to as the "bit duration", the duration of Tc is referred to as the "chip duration", and the reciprocals of these are referred to as the "bit rate" and "chip rate", respectively. The ratio of T to Tc (i.e. T/Tc) is referred to as the "spreading ratio".
The spectrum distribution of a spread-spectrum modulated signal exhibits the shape of a sinc function, as shown in FIG. 22. The bandwidth of a main lobe ML is equal to twice the chip rate (i.e. ML=2/Tc), and the bandwidth of a side lobe SL is 1/Tc. Since the PSK signal prior to spread-spectrum modulation is an ordinary PSK signal modulated at the bit rate 1/T, the occupied bandwidth is 2/T. Accordingly, if the occupied bandwidth of the spread-spectrum modulated signal is made the bandwidth (=2/Tc) of the main lobe, the bandwidth of the original PSK-modulated signal will be broadened T/Tc times by applying spread-spectrum modulation. The energy is diffused as a result. FIG. 23 is an explanatory view illustrating the manner in which bandwidth is enlarged by spread-spectrum modulation. Shown in FIG. 23 are a narrow bandwidth-modulated signal NM and a spread-spectrum modulated signal SM.
FIG. 24 is a structural view illustrating the operating principle of a receiver in spread-spectrum communication. Shown in FIG. 24 are an antenna 5, a wide-band bandpass filter 6 for passing only signals of necessary frequency bands and preventing radio interference from unnecessary frequency bands, a de-spreading circuit 7, a narrow-band bandpass filter 8 and a detector circuit 9 such as a PSK demodulator. The de-spreading circuit 7 has a construction identical with that of the spreading circuit 2 on the transmitting side and includes a spreading code generator 7a for outputting a rectangular spreading code sequence the same as that on the transmitting side, and a multiplier 7b for multiplying the output signal of the bandpass filter 6, by the spreading code.
The wide-band reception signal sent to the receiver is restored to the original narrow-band modulated signal via the de-spreading circuit 7 similar to the spreading circuit on the transmitting side. This is followed by the generation of a baseband waveform via the detector circuit 9, which is of the ordinary type. The reason why the narrow-band modulated signal is obtained by the de-spreading circuit 7 is as set forth below.
As shown in FIG. 25, let a(t) represent the narrow-band modulated wave on the transmitting side, c(t) the spreading code sequence (spreading code) and x(t) the transmitted waveform. These are related as follows: EQU x(t)=a(t).multidot.c(t)
If attenuation and the effects of noise during transmission are neglected, the transmitted waveform x(t) arrives on the receiving side intact. The spreading code used by the de-spreading circuit 7 has a waveform exactly the same as that of the spreading code used in spread-spectrum modulation on the transmitting side, as mentioned above. Accordingly, the output y(t) of the de-spreading circuit 7 is given by the following equation: EQU y(t)=x(t).multidot.c(t)=a(t).multidot.c.sup.2 (t)
The output signal y(t) enters the bandpass filter 8. Passing this signal through the bandpass filter is the same as integrating the signal. Thus the output of the bandpass filter is given by the following equation: EQU .intg.y(t)dt=a(t).multidot..intg.c.sup.2 (t)dt
The integral on the right side of this equation is an autocorrelation value obtained when the shift in time is made zero. The autocorrelation value is unity. Accordingly, the output of the bandpass filter is a(t) and the narrow-band modulating signal is obtained.
Code division multiple access (CDMA) is a method of communication using a different spreading code for each channel or user, whereby the information transmitted on the respective channels is multiplexed by the codes. FIG. 26 is a diagram for describing the principle of CDMA on two channels. Shown in FIG. 26 are a transmitter TR in which CH1 is a first channel, CH2 a second channel and CMP a combining unit, and first and second receivers RV1, RV2, respectively.
An important point in CDMA is the "similarity" of the spreading codes used by each of the channels. When almost identical spreading codes are used by each of the channels, the channels interfere with each other severely. A so-called "correlation value" is a measure of the degree to which interference between channels occurs. The correlation value is defined by the following equation with respect to two waveforms a(t) and b(t): EQU R=.intg.a(t).multidot.b(t)dt T: period
The integration is carried out over one period T of a(t), b(t). We have R=1 when a(t) and b(t) are exactly identical waveforms and R=-1 when the waveforms are of opposite signs. On the average, looking at one period, the value of R obtained is zero when there is no relationship between the value of a(t) at a certain time and the value of b(t) at the same time.
Consider the first receiver RV1 in a situation where CDMA is performed using, as the spreading code, two waveforms c.sub.1 (t) and c.sub.2 (t) of such a combination that the correlation value R is zero. The signals from the first and second channels CH1 and CH2 arrive at the first receiver RV1. When the first receiver RV1 de-spreads the received signals using the code c.sub.1 (t), a bandpass filter 8.sub.1 outputs a signal represented by the following equation: EQU .intg.{a.sub.1 (t)c.sub.1 (t)c.sub.1 (t)+a.sub.2 (t)c.sub.2 (t)c.sub.1 (t)}dt
The .intg.{a.sub.2 (t)c.sub.2 (t)c.sub.1 (t)}dt part of this is zero because the correlation value between c.sub.2 (t) and c.sub.1 (t) is zero. Further, .intg.c.sub.1 (t)c.sub.1 (t)dt is unity since this is an autocorrelation value in which the displacement in time is zero. Accordingly, the output of the low-pass filter 8.sub.1 of the first receiver RV1 is al(t) and the influence of the signal making use of c.sub.2 (t) as the spreading code is entirely absent. The same is true for the second receiver RV2. This will hold even if the number of simultaneously connected communication channels is increased. However, it is required that the correlation value be zero for the spreading codes of all combinations.
In actual CDMA, mutual influence cannot be measured merely by the correlation value. The reason for this is that the transmitting parties do not emit radio waves at exactly the same timing (i.e. in synchronous fashion). Accordingly, the correlation values of c.sub.1 (t) and c.sub.2 (t) are not merely compared; it is required that the correlation values be observed for a case where c.sub.1 (t) and c.sub.2 (t) are shifted arbitrarily in time.
Accordingly, it is required that a base station handling a plurality of channels or a mobile station exhibiting a high transmission rate through use of a plurality of channels have a function for generating, amplifying and transmitting code-multiplexed signals. Code multiplexing is carried out by linear voltage addition of signals spread by codes, a code-multiplexed signal obtained by voltage addition is bandwidth-limited by a chip shaping filter, the bandwidth-limited code-multiplexed signal is converted to a radio frequency and then subsequently amplified by a power amplifier before being transmitted.
FIG. 27 is a diagram showing the construction of a prior-art CDMA transmitter which code-multiplexes and transmits data on a number of channels. As shown in FIG. 27, the transmitter includes serial/parallel (S/P) converters 11.sub.1 .about.11.sub.n for alternately distributing, one bit at a time, serial data D.sub.1 .about.D.sub.n of first through nth channels, respectively, thereby converting the data to I-component (in-phase component) data D.sub.ij (j=1,2, . . . n) and Q-component (quadrature-component) data D.sub.qj (j=1, 2, . . . n); spreading circuits 12.sub.1 .about.12.sub.n for multiplying the data D.sub.ij, D.sub.qj by spreading codes C.sub.ij, C.sub.qj, respectively; a combiner 13i for outputting an I-component code-multiplexed signal V.sub.I by combining the I-component spread-spectrum modulated signals output by the respective spreading circuits 12.sub.1 .about.12.sub.n ; a combiner 13q for outputting a Q-component code-multiplexed signal V.sub.Q by combining the Q-component spread-spectrum modulated signals output by the respective spreading circuits 12.sub.1 .about.12.sub.n ; chip shaping filters 14i, 14q for limiting the bandwidth of the code-multiplexed signals V.sub.I, V.sub.Q, respectively; DA converters 15i, 15q for converting the digital outputs of the respective filters 14i, 14q to analog signals; a quadrature modulator 16 for applying quadrature modulation to the code-multiplexed signals V.sub.I, V.sub.Q of the I and Q components; and a power amplifier 17 for amplifying the output of the quadrature modulator and entering the amplified signal into an antenna, not shown.
The quadrature modulator 16 includes a carrier generator 16a for outputting a carrier wave cos.omega.t having a prescribed frequency, a 90.degree. phase shifter 16b for shifting the phase of the carrier wave by 90.degree. and outputting -sin.omega.t, a multiplier 16c for multiplying the output signal of the DA converter 15i by cos.omega.t, a multiplier 16d for multiplying the output signal of the DA converter 15q by -sin.omega.t, and a combiner 16e for combining the outputs of the multipliers 16c and 16d.
In CDMA, the amplitude (the outputs of the combiners 13i, 13q in FIG. 27) of the code-multiplexed signals is the sum of the voltages of the number of items of information multiplexed (the number of channels), and therefore maximum power Pmax is proportional to the square of the number of multiplexed channels. More specifically, the output of each of the spreading circuits is either +1 or -1, and the maximum amplitude of the code-multiplexed signal prevailing when +1 is being output by all spreading circuits of the n channels is n. The maximum power is proportional to n.sup.2. Mean power Pmean is proportional to the number n of multiplexed channels. This means that the peak factor (=Pmax/Pmean) of a code-multiplexed signal in a case where the number n of multiplexed channels is large becomes very large.
In wireless communication, the frequency band used in communication is limited. Consequently, it is necessary to suppress broadening of the frequency spectrum caused by non-linear distortion in the power amplifier (FIG. 27). More specifically, since broadening of the frequency spectrum is a cause of interference between adjacent channels, it is required that such broadening be reduced. Owing to this requirement, it is required that operation be performed in a linear region in a case where the code multiple signal is amplified by a power amplifier. A large output back-off must be employed. When output back-off is made large, however, a problem which arises is a large decrease in the power efficiency of the power amplifier. On the other hand, if the output back-off is not made sufficiently large, broadening of the spectrum is caused by non-linear distortion in the power amplifier. The problem which arises in this case is a decline in the frequency utilization efficiency of the system.
FIG. 28 shows an example of an AM-AM characteristic (input power vs. gain characteristic) of a power amplifier, and FIG. 29 shows an example of an AM-PM characteristic (input power vs. phase characteristic) of a power amplifier. The gain characteristic and phase characteristic of a power amplifier are flat and so is the input/output characteristic as long as the input power is small. There is also no phase rotation under these conditions. However, when the input power exceeds a certain level, gain starts to decline, a phase lag develops and each characteristic becomes non-linear. The output power level at which gain declines by 1 dB is referred to as the "1 db compression level", and the difference between this level and the mean output power is the output back-off OBO.
Even if the mean power level of the input signal resides in the linear area in such a non-linear amplifier, a signal having the maximum power level or a level near this level surpasses the 1 dB compression level owing to balance between the output back-off OBO and peak factor, distortion is produced and the frequency spectrum broadens. Since the peak factor is very large in a CDMA transmitter, as mentioned above, this problem is a serious one.
If the output back-off OBO is enlarged by lowering the mean power level of the input signal in such a manner that the 1 dB compression level will not be exceeded when the input signal of the maximum output level arrives, distortion does not develop and there is no broadening of the frequency spectrum. However, lowering the mean output level causes a decline in the power efficiency of the power amplifier.
Thus, in the prior art, when distortion in a power amplifier or broadening of the frequency spectrum is prevented by lowering the mean power level of the input signal (for which the output back-off OBO is large), the power efficiency of the power amplifier falls. Conversely, if the efficiency of a power amplifier is raised by raising the mean power level of the input signal (for which the output back-off OBO is small), then distortion is produced in the power amplifier and the frequency spectrum widens.
Further, as shown in FIG. 27, the DA converters 15i, 15q are required in the arrangement for producing the code-multiplexed signal by digital signal processing. There is a limitation upon the number of quantization bits in such DA converters and full scale is set in such a manner that the maximum value of the code-multiplexed signal can be delivered as an output. With a CDMA transmitter, however, the peak factor of the code-multiplexed signal is very large. As a result, the number of effective bits with regard to a signal in the vicinity of mean power having a high frequency of occurrence decreases and quantization noise increases. The deterioration due to quantization has a deleterious effect upon the noise floor of the spectrum frequency and is a cause of interference between adjacent channels.